EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Within the framework of the development of ceramic based on mixed composition (Al2O3 - mullite), the formulation of the raw materials involves the kaolinite Al2Si2O5(OH)4 (phyllosilicate) as a source of silicon. Natural kaolinite generally crystallizes in the form of pseudo-hexagonal platelets of small thickness (from 1 µm to 100 nm). The shape of these grains, composed of stacks of several sheets associated with a structural anisotropy, promotes complex surface properties with faces/edges of aluminous (layers of octahedrons AlO2(OH)4) or siliceous (layers of tetrahedrons SiO4) types. A patch model based on the dissociating the contributions of surface charges attributed to the basal and lateral surfaces of the layers, better explains the physicochemical phenomena occurring at the interface between the grains and the aqueous solution. Both density of reactive sites with water molecules combined with the ratio between the sites density on the basal and lateral surfaces and the crystallinity of the sheets influence the electrical charges developed onto the grains surface and thus their dispersion. This research work has consisted in characterizing by acoustophoresis, the electrokinetic properties of two kaolin powders, mainly composed of kaolinite but exhibiting different particle size distributions and specific surface of grains. The results show that this technique is highly sensitive to the crystallochemistry of kaolinite ; specific molecules also allow to characterize the reactivity of the basal and lateral faces according to the pH and ionic strength conditions. The rheological properties of concentrated suspensions were also studied and related to the electrokinetic properties of diluted suspensions.